It has been known for some time that the WMAP data is more consistant with the existence of four neutrino species than three. Nevertheless, most cosmologists shrug this off as three is by no means ruled out. However, Hamann et al. 2010 demonstrate that such a dismissal may be a mistake.
It turns out, when WMAP 7 year data is combined with Sloan data, the three neutrino species model is ruled out by nearly two sigma. The best fit number of neutrino species becomes 4.78 +/- 1.79 at 95% confidence. Furthermore, big bang nucleosynthesis (BBN) data involving Heluim abundances seems to confirm that such an excess better fits the data.
With this in mind, Hamann et al. 2010 decides to test just how many extra neutrinos are needed to fit the combined data of "the WMAP 7-year data release, small-scale CMB observations from ACBAR, BICEP and QuAD, the 7th data release of the Sloan Digital Sky Survey, and measurement of the Hubble parameter from Hubble Space Telescope observations". Their findings are plotted above. They confirm that when all data is added together, the existence of one or two extra neutrinos provides a much better fit than only the standar three.
If this is real it would be major news! On one hand such a "4th" or even "5th" neutrino would have to exist at low energies as it has clearly affected both BBN and CMB physics. However, extra neutrinos at such low energies have alluded modern particle accelerators. Therefore, such much neutrinos must be "sterile" in that they do not couple to the rest of the standard model the way normal neutrinos do. Furthermore, they must not have a lepton partner the same way other neutrinos do. (Example: like the electron neutrino does with the electron.)
Interestingly, if there are an extra one or two of such neutrinos in nature, the Planck satellite has a good chance of making a 5-sigma discovery! (See plot below). If this happens, the discovery of such interesting low energy neutrinos could well go down as one of Planck's greatest contributions to science.
Jan Hamann, Steen Hannestad, Georg G. Raffelt, Irene Tamborra, & Yvonne Y. Y. Wong (2010). Cosmology seeking friendship with sterile neutrinos Eprint arXiv: 1006.5276v1